HashMap中modCount屬性用實(shí)現(xiàn)fail-fast機(jī)制，即遍歷過(guò)程如果有增刪改，則馬上拋出異常。每次增刪改modCount加一。
KeySet和Values中都有forEach方法。

public final void forEach(Consumer<? super K> action) {
            Node<K,V>[] tab;
            if (action == null)
                throw new NullPointerException();
            if (size > 0 && (tab = table) != null) {
                int mc = modCount;
                for (Node<K,V> e : tab) {
                    for (; e != null; e = e.next)
                        action.accept(e.key);
                }
                if (modCount != mc)
                    throw new ConcurrentModificationException();
            }
        }

代碼可知forEach是全部遍歷完才比較，所以這個(gè)不算是fail-fast。
KeySet和Values中Iterator<K> iterator() 方法實(shí)現(xiàn)Iterator接口。

final Node<K,V> nextNode() {
            Node<K,V>[] t;
            Node<K,V> e = next;
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (e == null)
                throw new NoSuchElementException();
            if ((next = (current = e).next) == null && (t = table) != null) {
                do {} while (index < t.length && (next = t[index++]) == null);
            }
            return e;
        }

        public final void remove() {
            Node<K,V> p = current;
            if (p == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            current = null;
            removeNode(p.hash, p.key, null, false, false);
            expectedModCount = modCount;
        }

expectedModCount初始化為modCount

nextNode()中會(huì)先判斷是否變化，變化則直接fail-fast

remove()比較特殊，先判斷是否變化，變化則直接fail-fast。然后刪除，這時(shí)modCount會(huì)變化。最后再expectedModCount = modCount;這樣避免了fail-fast。

所以如果遍歷同時(shí)修改則必須調(diào)用iterator的remove方法才可以。
如果遍歷時(shí)直接調(diào)用HashMap的remove方法會(huì)拋出異常。
forEach雖然也會(huì)拋出異常，但是并不是fail-fast。